Preparation of yttria-gadolinia ceramic scintillators by vacuum hot pressing

ABSTRACT

Polycrystalline ceramic scintillators are prepared by a vacuum hot-pressing method. The process includes pressing a multi-component powder at high temperature under vacuum. Following a holding period, the pressure and temperature are increased and maintained for a predetermined length of time. The finished scintillator includes Y 2  O 3 , Gd 2  O 3 , and one or more of Eu 2  O 3 , Nd 2  O 3 , Yb 2  O 3 , DY 2  O 3 , Pr 2  O 3 , and Tb 2  O 3  rare earth activator oxides. The finished scintillator may also include at least one of SrO and CaO as afterglow reducers.

RELATED APPLICATIONS

This application is related to application Ser. Nos. 389,812, 389,814,389,815, 389,817, 389,818, 389,828, 389,829 and 389,830, all filed onthe same date and all assigned to the same assignee as the presentinvention.

BACKGROUND OF THE INVENTION

The present invention relates to rare-earth doped ceramic scintillatorsfor computerized tomography (CT) and other x-ray, gamma radiation, andnuclear radiation detecting applications. More specifically, theinvention relates to rare-earth-doped, polycrystalline, yttria/gadolinia(Y₂ O₃ /Gd₂ O₃) ceramic scintillators.

Computerized tomography scanners are medical diagnostic instruments inwhich the subject is exposed to a relatively planar beam or beams ofx-ray radiation, the intensity of which varies in direct relationship tothe energy absorption along a plurality of subject body paths. Bymeasuring the x-ray intensity (i.e., the x-ray absorption) along thesepaths from a plurality of different angles or views, an x-ray absorptioncoefficient can be computed for various areas in any plane of the bodythrough which the radiation passes. These areas typically compriseapproximately a square portion of about 1 mm×1 mm. The absorptioncoefficients are used to produce a display of, for example, the bodilyorgans intersected by the x-ray beam.

An integral and important part of the scanner is the x-ray detectorwhich receives the x-ray radiation which has been modulated by passagethrough the particular body under study. Generally, the x-ray detectorcontains a scintillator material which, when excited by the impingingx-ray radiation, emits optical wavelength radiation. In typical medicalor industrial CT applications, the optical output from the scintillatormaterial is made to impinge upon photoelectrically responsive materialsin order to produce electrical output signals, the amplitude of which isdirectly related to the intensity of the impinging x-ray radiation. Theelectrical signals are digitized for processing by digital computermeans which generates the absorption coefficients in a form suitable fordisplay on a cathode ray tube screen or other permanent media.

Due to the specific and demanding computerized tomography requirements,not all scintillator materials which emit optical radiation uponexcitation by x-ray or gamma ray radiation are suitable for CTapplications. Useful scintillators must be efficient converters of x-rayradiation into optical radiation in those regions of the electromagneticspectrum (visible and near visible) which are most efficiently detectedby photosensors such as photomultipliers or photodiodes. It is alsodesirable that the scintillator transmit the optical radiationefficiently, avoiding optical trapping, such that optical radiationoriginating deep in the scintillator body escapes for detection byexternally situated photodetectors. This is particularly important inmedical diagnostic applications where it is desirable that x-ray dosagebe as small as possible to minimize patient exposure, while maintainingadequate quantum detection efficiency and a high signal-to-noise ratio.

Among other desirable scintillator material properties are shortafterglow or persistence, low hysteresis, high x-ray stopping power, andspectral linearity. Afterglow is the tendency of the scintillator tocontinue emitting optical radiation for a time after termination ofx-ray excitation, resulting in blurring, with time, of theinformation-bearing signal. Short afterglow is also highly desirable inapplications requiring rapid sequential scanning such as, for example,in imaging moving bodily organs. Hysteresis is the scintillator materialproperty whereby the optical output varies for identical x-rayexcitation based on the radiation history of the scintillator. This isundesirable due to the requirement in CT for repeated precisemeasurements of optical output from each scintillator cell and where theoptical output must be substantially identical for identical x-rayradiation exposure impinging on the scintillator body. Typical detectingaccuracies are on the order of one part in one thousand for a number ofsuccessive measurements taken at relatively high rate. High x-raystopping power is desirable for efficient x-ray detection. X-rays notabsorbed by the scintillator escape detection. Spectral linearity isanother important scintillator material property because x-raysimpinging thereon have different frequencies. Scintillator response mustbe substantially uniform at all x-ray frequencies.

Among scintillator phosphors considered for CT use are monocrystallinematerials such as cesium iodide (CsI), bismuth germanate (Bi₄ Ge₃ O₁₂),cadmium tungstate (CdWO₄), and sodium iodide (NaI). Many of theaforementioned materials typically suffer from one or more deficienciessuch as excessive afterglow, low light output, cleavage, low mechanicalstrength, hysteresis, and high cost. Many monocrystalline scintillatorsare also subject to hygroscopic attack. Known polycrystallinescintillators are efficient and economical. However, due to theirpolycrystalline nature, such materials are not efficient lightpropagators and are subject to considerable optical trapping. Internallight paths are extremely long and tortuous, resulting in unacceptableattenuation of optical output.

Fabrication of monocrystalline scintillators from multicomponent powderconstituents is typically not economical and is frequently impractical.The multicomponent powder composition must be heated to a temperatureabove its melting point, and ingots of dimensions larger than those ofeach detector channel are grown from the melt. Considering the size ofthe bars required and the temperatures involved, the process isdifficult in and of itself. In addition, some materials exhibit phasechanges while cooling, which would cause the crystals to crack whencooled after the growing process. Furthermore, single crystals tend tobe susceptible to the propogation of lattice defects along the crystalplanes.

U.S. Pat. No. 4,242,221 issued to D. A. Cusano et al (assigned to thesame assignee as the present invention) describes methods forfabricating polycrystalline phosphors into ceramic-like scintillatorbodies for use in CT.

The present invention provides improved ceramic scintillators composedof yttria-gadolinia and including a variety of rare earth activators forenhancing luminescent efficiency.

The terms "transparency" and "translucency", as used herein, describevarious degrees of optical clarity in the scintillator material.Typically, the inventive scintillator materials exhibit an opticalattenuation coefficient of less than 100 cm⁻¹, as measured by standardspectral transmittance tests (i.e., "narrow" angle transmission) on apolished scintillator material plate, at the luminescent wavelength ofthe respective ion. The most desirable scintillator materials have lowerattenuation coefficients and hence higher optical clarity(transparency).

SUMMARY OF THE INVENTION

The invention relates to a method for preparing a yttria-gadoliniaceramic scintillator body. The method includes the step of preparing amulticomponent powder containing between about 5 and 50 mole percent Gd₂O₃ and between about 0.02 and 12 mole percent of at least one rare earthactivator oxide selected from the group consisting of Eu₂ O₃, Nd₂ O₃,Yb₂ O₃, Tb₂ O₃, and Pr₂ O₃, the remainder of the multicomponent powderbeing Y₂ O₃. The multicomponent powder is pressed under vacuum at afirst temperature and pressure. The temperature and pressure are thenincreased and maintained so as to form the polycrystalline ceramicscintillator body.

The powder may also include at least one of SrO and CaO as afterglowreducers.

It is an object of the invention to provide a vacuum hot pressingprocess for preparing rare-earth-doped, transparent-to-translucentpolycrystalline yttria-gadolinia ceramic scintillators having cubiccrystalline structure, high x-ray stopping power, high density, highuniformity, and high radiant efficiency, and which are useful inradiation detectors such as those used in CT and digital radiography.

It is another object of the invention to provide a vacuum hot pressingprocess for preparing polycrystalline yttria-gadolinia ceramicscintillators exhibiting low luminescence afterglow.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are set forth withparticularity in the appended claims. The invention itself, however,both as to its organization and method of operation, together withfurther objects and advantages thereof, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings.

FIG. 1 is a graph depicting the effect of increased thoria (ThO₂)content on the light output of a yttria-gadolinia ceramic scintillatorcontaining 3 mole percent Eu₂ O₃.

FIG. 2 is a graphical illustration of the dependence of the relativelight output on Eu₂ O₃ activator concentration in an inventivescintillator material containing 25 mole percent Gd₂ O₃.

FIG. 3a illustrates graphically the dependence of scintillatorefficiency on yttria-gadolinia compositional ratio of an inventiveceramic scintillator containing 3 mole percent Eu₂ O₃.

FIG. 3b is a graph illustrating 73 kev x-ray stopping power versusyttria-gadolinia compositional ratio of an inventive ceramicscintillator.

FIG. 4 is a graphical illustration of the effect of increased Yb₂ O₃concentrations on scintillator material afterglow.

FIG. 5 depicts graphically the relative light output of an inventivescintillator material with increased Yb₂ O₃ content.

FIG. 6 is a graphical illustration of the effect of increased SrOconcentration on scintillator material afterglow.

DETAILED DESCRIPTION OF THE INVENTION

U.S. Pat. No. 3,640,887, issued to R. C. Anderson and assigned to thesame assignee as the present invention, describes the manufacture oftransparent polycrystalline ceramic bodies. The bodies include theoxides of thorium, zirconium and hafnium and mixtures thereof withoxides of rare earth elements 58 through 71 of the Periodic Table. Thebodies may optionally include yttria. The average ionic radius of therare earth oxide, with or without yttria, must not exceed about 0.93 Å,and the difference in ionic sizes of the constituent oxides should notexceed about 0.22 Å. The polycrystalline ceramics are used in hightemperature applications and/or in applications requiring lighttransmission. Exemplary applications include high temperaturemicroscopes, lamp envelopes, laser applications, and furnace windows.

The aforedescribed patent teaches that each polycrystalline ceramic bodyincludes between about 2 to 15 mole percent of thoria (ThO₂), zirconia(ZrO₂), hafnia (HfO₂), or some combination thereof to act as adensifying agent during sintering.

The inventors herein have found, however, that the inclusion of ThO₂,ZrO₂, or HfO₂ in the scintillator materials of the present invention inthe quantities specified in the aforementioned Anderson patent resultsin a material having greatly reduced light output when excited by highenergy radiation such as x-rays, making such materials unsuitable for CTapplications. FIG. 1 illustrates relative light output (vertical axis)of a polycrystalline ceramic composed of about 58.7 mole percent Y₂ O₃,38 mole percent gadolinia, 3 mole percent Eu₂ O₃, and 0.3 mole percentYb₂ O₃, with increasing ThO₂ mole percent (shown on the horizontalaxis). As the quantity of ThO₂ is increased, the quantity of Y₂ O₃ iscorrespondingly decreased. The average ionic radius of the ceramicconstituents and the difference between ionic radii are as specified inthe Anderson patent. It is evident from FIG. 1 that the light output fora material containing 2 mole percent ThO₂ (the minimum amount specifiedby Anderson) is only 5 percent of the light output for the same materialwithout thoria. In fact, the addition of as little as 0.5 mole percentTHO₂, well below the lower limit specified in the Anderson patent,reduces the light output to a value of about 18 percent of that measuredfor the material without thoria. Ceramic scintillator materials usefulin CT should have a light output no lower than about 35 percent of thelight output of the material without ThO₂.

Substantial light output decreases have also been observed for additivessuch as cerium oxide (CeO₂), titanium oxide (TiO₂), zirconium oxide(ZrO₂), and tantalum oxide (Ta₂ O₅). For example, the relative lightoutput of a ceramic body containing 55.5 mole percent Y₂ O₃, 38 molepercent Gd₂ O₃, 1 mole percent Eu₂ O₃, 0.5 mole percent Yb₂ O₃, and 2mole percent ZrO₂ was found to be 4 percent of the light output of thesame material without ZrO₂. These tetravalent (4⁺) and pentavalent (5⁺)additive species have an inhibiting effect on light output. It isimportant to note that light output refers to scintillation resultingfrom x-ray excitation. This is a significant distinction since someceramic bodies fluoresce under ultraviolet excitation, but do notscintillate upon x-ray excitation, for example.

In accordance with the present invention, translucent-to-transparentrare-earth ceramic scintillator bodies are produced without the additionof the aforedescribed scintillation inhibiting densifying additives. Thelight output of the inventive ceramic scintillators upon x-rayexcitation is sufficient to render them useful in CT.

The inventive scintillators are made up of rare earth yttria-gadoliniahosts and trivalent rare earth activator oxides. The scintillator bodiesare fabricated by one of several methods, such as sintering, sinteringwith gas hot isostatic pressing (GHIP), and hot pressing (all more fullydescribed hereinafter). The finished scintillator bodies having optimumover-all properties are comprised of a cubic solid solution of thechemical constituents as verified by x-ray diffraction techniques.

The oxides of trivalent rare earth elements such as europium, neodymium,ytterbium, dysprosium, terbium, and praseodymium (Eu₂ O₃, Nd₂ O₃, Yb₂O₃, Dy₂ O₃, Tb₂ O₃, and Pr₂ O₃, respectively) are added to the basicyttria-gadolinia system as activators to enhance scintillatorefficiency. Yttria-gadolinia scintillators containing Eu₂ O₃ exhibitexcellent scintillating efficiency. Generally, rare earth activatorconcentration may range between 0.02 and 12 mole percent. Optimumconcentration of Eu₂ O₃ is between 1 and 6 mole percent. This isillustrated in FIG. 2 which shows that the highest relative lightoutput, indicated on the vertical axis, is observed for Eu₂ O₃concentrations of between about 1 and 6 mole percent, as indicated onthe horizontal axis. The curve depicted in FIG. 2 was obtained byvarying Eu₂ O₃ content of a scintillator material containing 25 molepercent Gd₂ O₃, the remainder being Y₂ O₃.

An exemplary yttria-gadolinia scintillator material along neodymiumoxide (Nd₂ O₃) activator is made up of 30 mole percent Gd₂ O₃, 0.25 molepercent Nd₂ O₃, the remainder being Y₂ O₃. Nd₂ O₃ is preferably added inquantities of between 0.05 and 1.5 mole percent. Most preferably,however, Nd₂ O₃ is added in concentrations of between 0.1 and 0.5 molepercent. Preferred terbium oxide (Tb₂ O₃) activator concentration isbetween 0.05 and 3.0 mole percent, while the preferred concentration ofdysprosium oxide (Dy₂ O₃) activator is between 0.03 and 1.0 molepercent. Exemplary compositions of yttria-gadolinia scintillatormaterials employing Tb₂ O₃ and Dy₂ O₃ rare earth activators comprise0.15 mole percent Tb₂ O₃, and 0.2 mole percent Dy₂ O₃, respectively,each including 40 mole percent Gd₂ O₃, the remainder being Y₂ O₃. Thepreferred range for Yb₂ O₃ activator is between about 0.10 and 2.0 molepercent. The preferred mole percentage for the Pr₂ O₃ activator isbetween 0.02 and 0.05.

It is to be noted that activator efficacy is independent of thecompositional ratios of Y₂ O₃ and Gd₂ O₃. Eu₂ O₃ is the preferredactivator followed, in order of preference, by Nd₂ O₃ and Dy₂ O₃.

FIG. 3a illustrates the dependence of scintillator efficiency, asmeasured by relative light output, on the relative mole percent contentsof yttria and gadolinia in a scintillator material containing 3 molepercent Eu₂ O₃. The relative mole percentages of Gd₂ O₃ and Y₂ O₃ areshown on the horizontal axis, while the relative light output is shownon the vertical axis. The dashed line at 50 mole percent Gd₂ O₃ and 50mole percent Y₂ O₃ indicates the beginning of a gradual crystallinephase transition in the scintillator material structure from the cubicphase to the monoclinic phase. It will be observed that high relativelight output is obtained from scintillator materials containing up toabout 50 mole percent Gd₂ O₃. Scintillator materials containing between50 and 65 mole percent Gd₂ O₃ exhibit modest relative light output, butare increasingly subject to grain boundary cracking and reduced relativelight output due to progressive transition from cubic to monocliniccrystalline phase.

The cubic crystalline phase is characterized by a high degree ofscintillator material structural symmetry. Materials having suchstructure are particularly desirable for CT applications. Scintillatormaterials having increasing amounts of monoclinic phase arecharacterized by lower relative light outputs and poor optical claritydue to grain boundary cracking and nonuniform crystalline structure.Materials having such noncubic structure exhibit appreciable lightscattering and reabsorption due to a longer effective relative lightpath length, thereby decreasing the amount of light available fordetection by external photosensors.

In considering the usefulness of the scintillator material in CTapplications, the x-ray stopping power of the material must also beconsidered. FIG. 3b illustrates the dependence of 73 kev x-ray stoppinglength on yttria-gadolinia compositional ratio for transparent andefficient scintillators. Stopping power is measured in terms of x-raystopping length, i.e., the distance an x-ray photon penetrates into thescintillator prior to its conversion to optical wavelength photons whichare detectable by photosensors. X-ray stopping length is primarilydependent on Gd₂ O₃ content and, as shown in FIG. 3b, increases withincreased Gd₂ O₃ concentration. Generally, it is preferred to usebetween about 5 mole percent and 50 mole percent Gd₂ O₃. Materialscontaining less than about 5 mole percent Gd₂ O₃ exhibit low x-raystopping power for most practical detector design, while materialshaving more than 50 mole percent are increasingly non-cubic and exhibitpoor optical clarity. A more preferred range of Gd₂ O₃ content isbetween 20 and 40 mole percent. The most preferred range of Gd₂ O₃concentration is between 30 mole percent and 40 mole percent,corresponding to an x-ray stopping length of about 0.45 mm. For a 2millimeter thick scintillator material having an x-ray stopping lengthof 0.45 mm, approximately 99 percent of x-ray photons entering thematerial are converted to optical wavelength photons.

Certain additives are useful in the yttria-gadolinia scintillator systemof the present invention to reduce undesirable scintillator materialluminescent afterglow, which may lead to undesirable distortion and thepresence of artifacts in reconstructed images. The luminescent afterglowphenomenon is classifiable into primary or fundamental afterglow andsecondary afterglow. Primary afterglow is of relatively short duration(up to approximately 3 milliseconds), while secondary afterglow may beseveral times to much more than several times the primary decay time.Fundamental luminescent afterglow of a phosphor is thought to beinextricably associated with the specific activator identity and theactivator local environment in the host matrix (in this caseyttria-gadolinia). The secondary, and most objectionable type ofafterglow, can be associated with more subtle changes in the activatorenvironment or simply with the presence of additional electron-hole"trapping" centers created by native defects and/or low level impuritiesat other sites in the host crystal. Both types of afterglow may bereduced by suitable purification or the addition of compensatingdopants. The added dopants used to reduce afterglow do so by forming"killer" centers which are believed to compete with the activatorcenters for electron-hole pairs that otherwise combine radiatively atthe activator centers.

Luminescent afterglow of rare earth doped yttria-gadolinia ceramicscintillators of the present invention can be substantially reduced byseveral types of additives.

The addition of ytterbium oxide (Yb₂ O₃), itself a luminescent activatorin the yttria-gadolinia host if used alone as described heretofore,results in the reduction of undesirable secondary afterglow with onlyminor sacrifice of luminescent efficiency. If, as depicted in FIG. 4,the mole percentage of Yb₂ O₃ is increased from zero to about 2 molepercent, the primary or fundamental afterglow, τ, of the scintillatormaterial activated with 3 mole percent of Eu₂ O₃ is reduced from 1.1 to0.82 milliseconds. An increase in Yb₂ O₃ content from 0 to 2 molepercent is accompanied by the loss of nearly 50 percent of scintillatormaterial luminescent efficiency as graphically depicted in FIG. 5, inwhich relative light output is shown on the vertical axis, while the Yb₂O₃ concentration is shown on the horizontal axis.

Curves A, B, C, and D, depicted in FIG. 4, illustrate the fraction ofsecondary luminescent afterglow (vertical axis) remaining at timesgreater than 10 milliseconds (horizontal axis) following the cessationof x-ray excition. For a scintillator material having 30 mole percentGd₂ O₃, 3 mole percent Eu₂ O₃, and 67 mole percent Y₂ O₃, but no Yb₂ O₃,it is evident from Curve A that about three percent of the luminescencepresent immediately upon x-ray shut-off remains at the end of 10milliseconds following x-ray turn-off. Curves B, C, and D depictfractional afterglow for similar scintillator materials whichadditionally contain 0.2, 0.5, and 2 mole percent Yb₂ O₃, respectively,and correspondingly less Y₂ O₃. It is apparent that increasingquantities of Yb₂ O₃ reduce secondary afterglow. For example, at about10 milliseconds after x-ray turn-off, fractional afterglow for ascintillator material containing 2 mole percent Yb₂ O₃ (Curve D) is onlyapproximately 0.7 percent (7×10⁻³) of its value immediately upontermination of x-ray excitation as compared to about 3 percent (3×10⁻²)for a material without Yb₂ O₃ (Curve A). The addition of 0.3 molepercent Yb₂ O₃ to a scintillator composition made up of 66.7 molepercent Y₂ O₃, 30 mole percent Gd₂ O₃, and 3 mole percent Eu₂ O₃ resultsin an extremely useful CT scintillator material having a fast decaytime. Preferred concentration of Yb₂ O₃ for afterglow reduction isbetween about 0.15 and 0.7 mole percent.

Another additive dopant which is effective in reducing scintillatormaterial luminescent afterglow is strontium oxide (SrO). The addition ofSrO results primarily in the reduction of secondary afterglow withrelatively little sacrifice of luminescent efficiency. In theyttria-gadolinia scintillator system, the quantity of SrO generallyfound to be useful in reducing afterglow is between 0.1 and 2 molepercent. It is shown in FIG. 6 that an increase in the quantity of SrOfrom 0 to 2 mole percent has no appreciable effect on primary afterglow,τ, 1.08 milliseconds and 1.10 milliseconds, respectively. However, thereis appreciable effect on secondary afterglow as depicted by curves E andF. A scintillator material having 30 mole percent Gd₂ O₃, 2 mole percentEu₂ O₃, 68 mole percent Y₂ O₃, but no SrO (curve E) exhibits, at about150 milliseconds after x-ray turn-off, about 0.8 percent (8×10⁻²) of theluminescence present immediately after x-ray shut-off. Scintillatormaterials having the same composition (curve F) but including 2 molepercent SrO (and 2 mole percent less Y₂ O₃) exhibit, after the sameelapsed time, only about 0.03 percent (3×10⁻⁴) fractional afterglow asindicated on the vertical axis in FIG. 6.

The addition of calcium oxide (CaO) to the scintillator material(instead of SrO) has been found to produce an afterglow reducing effectsimilar to that obtained with SrO. CaO is useful as an afterglow reducerin quantities of between 0.1 and 2 mole percent.

The aforedescribed yttria-gadolinia rare-earth-doped ceramicscintillator materials may be prepared by sintering, sintering plus gashot isostatic pressing, and hot pressing ceramic methods. The ceramicscintillator materials are preferably and most economically fabricatedby employing a sintering process.

A preliminary step in the fabrication of the ceramic scintillators, byany of the aforementioned methods, requires the preparation of asuitable powder containing the desired scintillator materialconstituents. In accordance with a first method for preparing such apowder, submicron-to-micron powders of yttria (Y₂ O₃) and gadolinia (Gd₂O₃) having purities of, for example, 99.99 percent to 99.9999 percentare mixed with the desired rare earth activators in the form of oxides,oxalates, carbonates, or nitrates and mixtures thereof. The mixing ofthe selected constituents may be carried out in an agate mortar andpestle or in a ball mill using water, heptane, or an alcohol (such asethyl alcohol) as liquid vehicles. Dry milling may also be used for bothmixing and breakup of powder aggregates. If dry milling is employed, agrinding aid such as 1 to 5 weight percent of stearic acid or oleic acidshould be employed to prevent powder packing or sticking inside the ballmill. A transparency promoter SrO may also be added in the form of anoxide, nitrate, carbonate, or oxalate before ball milling. If thevarious additives are nitrates, carbonates, or oxalates, a calciningstep is required to obtain the corresponding oxides prior to fabricationof the ceramic scintillator by any of the methods described hereinafter.

A second approach to obtaining the desired scintillator starting powderemploys a wet chemical oxalate method. In this method, the selectedmolar percentages of the nitrates of predetermined ones of Y, Gd, Eu,Nb, Yb, Dy, Tb, Pr, and Sr are dissolved in water and coprecipitated inoxalic acid to form the respective oxalates. The oxalate precipitationprocess involves the addition of an aqueous nitrate solution of thedesired scintillator material constituents to an oxalic acid solutionwhich is, for example, 80 percent saturated at room temperature. Theresulting coprecipitated oxalates are washed, neutralized, filtered, anddried in air at about 100° C. for approximately eight hours. Theoxalates are then calcined in air (thermally decomposed) atapproximately 700° C. to about 900° C. for a time ranging from one tofour hours, to form the corresponding oxides. Typically, heating for onehour at 800° C. is sufficient. Preferably, if either the hot pressing orthe sintering method is used to prepare the scintillator, the oxalatesand/or the resulting oxides are milled by one of several methods such asball, colloid, or fluid energy milling to enhance the optical clarity.Milling of the powder for between one-half hour and ten hours has beenfound to be sufficient. It should be noted, however, that typically theoptical clarity of the scintillator is improved by milling the oxalatesand/or oxides regardless of the preparation method.

Following the preparation of the selected powder composition by one ofthe methods described above, in accord with the preparation ofscintillator materials by sintering, selected amounts of the powdercomposition are formed into powder compacts by either die pressing, ordie pressing followed by isostatic pressing to further increase greendensity. A die material which is inert with respect to the scintillatorconstituents is preferred to avoid undesired reactions andcontaminations. Suitable die materials include alumina, silicon carbide,and metals such as molybdenum, hardened steel, or nickel-based alloys.The powder compacts are formed by die pressing at pressures of betweenabout 3,000 psi and 15,000 psi. Alternatively, the die pressed powdercompacts may be isostatically pressed at between about 10,000 and 60,000psi to further increase powder compact green density. If any grindingaids or compaction aids (lubricants, such as waxes) have been used, anoxidation treatment to remove all organic additives can be employedprior to sintering.

During the sintering phase, the compacts are heated in a hightemperature tungsten furnace, for example, in vacuum or a reducingatmosphere such as a wet hydrogen atmosphere (dew point of about 23° C.,for example) at a rate of between approximately 100° C. per hour and700° C. per hour to the sintering temperature of between 1800° C. and2100° C. The sintering temperature is then held from 1 hour to about 30hours to cause extensive densification and optical clarity development.Upon termination of the sintering step, the compacts are cooled from thesintering temperature to room temperature over a period of time rangingfrom about 2 to 10 hours.

Sintered ceramic scintillators may also be prepared by a heatingsequence which includes a hold at a temperature lower than the finalsintering temperature. Typically, the powder compact is heated at a rateof between 300° C./hr and 400° C./hr to a holding temperature of betweenabout 1600° C. and 1700° C. The holding period may range from 1 hour to20 hours, following which the temperature is raised to between about1800° C. and 2100° C. for final sintering for between 1 hour and 10hours. The increase from the holding temperature to the final sinteringtemperature is at a rate of between about 25° C./hr and 75° C./hr. Apreferred heating sequence comprises heating the powder compact to aholding temperature of 1660° C. in five hours, holding this temperaturefor 10 hours, followed by heating to 1950° C. in 6 hours, and thensintering at 1950° C. for 2 hours.

A first ceramic scintillator composed of 59.85 mole percent Y₂ O₃, 40mole percent Gd₂ O₃, and 0.15 mole percent Tb₂ O₃, and a secondscintillator made up of 59.8 mole percent Y₂ O₃, 40 mole percent Gd₂ O₃,and 0.2 mole percent Dy₂ O₃ were prepared using the preferred sinteringsequence. In each case, oxalates of the selected constituents wereprepared by the oxalate coprecipitation method and calcined at 800° C.for one hour to obtain the corresponding oxides. The oxides wereinitially cold pressed into powder compacts at 3500 psi and thenisostatically pressed at 29,000 psi to further increase green density.This was followed by heating the powder compacts in a wet H₂ atmospherein five hours to 1660° C. Following a 10 hour holding period at 1660° C.the temperature was raised to 1950° C. and held for two hours. Theresulting ceramic scintillators were then furnace-cooled to roomtemperature.

Yttria/gadolinia ceramic scintillators for luminescent applications mayalso be prepared by a combination of processes involving sintering andgas hot isostatic pressing (GHIP). The starting oxide powdercompositions are prepared in accordance with one of the aforedescribedmethods. Preferably, the oxalate coprecipitation method is used. By wayof example and not limitation, one useful yttria-gadolinia scintillatorcomposition comprised 66.7 mole percent Y₂ O₃, 30 mole percent Gd₂ O₃, 3mole percent Eu₂ O₃, and 0.3 mole percent Yb₂ O₃. Another usefulcomposition comprised 49.7 mole percent Y₂ O₃, 45 mole percent Gd₂ O₃, 5mole percent Eu₂ O₃, and 0.3 mole percent Yb₂ O₃. In contrast to thepreviously described sintering process, which preferably requiresmilling of the oxalate and/or oxide powders to produce transparentceramics, the process of sintering combined with GHIP permits thefabrication of transparent ceramics from unmilled powders.

In the fabrication of yttria-gadolinia ceramic scintillators by thecombined processes of sintering and gas hot isostatic pressing,following the preparation of a powder having the desired composition,powder compacts are formed by cold pressing at pressures of between3,000 psi and 10,000 psi, followed by isostatic pressing at pressures ofbetween 15,000 psi and 60,000 psi. The pressed compacts are thenpresintered to 93 to 98 percent of their theoretical density oftemperatures of about 1500° C. to 1700° C. for between 1 and 10 hours.The presintered compacts are then gas hot isostatically pressed withargon gas at pressures of 1,000 psi to 30,000 psi at temperaturesbetween 1500° C. and 1800° C. for 1 to 2 hours.

In accordance with an example of the preparation of a ceramicscintillator employing the sintering and GHIP technique, a powdercompact was formed by cold pressing approximately 20 grams of powder ina rectangular die at a pressure of approximately 4,000 psi. The samplewas then isostatically pressed at 30,000 psi to increase green densityto 49 percent of its theoretical value. The cold pressing of the samplewas followed by sintering in a wet hydrogen atmosphere (dew point 23°C.) for two hours at 1660° C. so that closed porosity was developed. Thedensity of the sintered sample, as measured by the water immersionmethod, at this stage in the fabrication process was determined to bebetween 93 and 98 percent of its theoretical value. In order to obtainadditional densification and optical transparency, the sample was gashot isostatically pressed in a carbon resistance furnace at 1750° C. forone hour at an argon pressure of 25,000 psi. During gas hot isostaticpressing, the temperature was increased to the final value of 1750° C.in a step-wise manner. The sample was initially heated in one hour to1400° C. and the temperature raised thence to 1750° C. in another hour.Following a holding period of one hour at 1750° C., the resultingceramic scintillator had a black appearance due to reduction in thereducing furnace atmosphere. The sample was rendered transparent tovisible light by suitable oxidation treatment such as heating in air ata temperature of 1200° C. for thirty-two hours. Comparison of thephysical dimensions of the sample before and after the GHIP treatmentindicates that the sample shrunk during the GHIP step, indicatingfurther densification. The finished ceramic exhibited a density ofgreater than 99.9 percent of theoretical value.

Transparent yttria-gadolinia ceramic scintillators may also be preparedby vacuum hot pressing a scintillator material powder prepared,preferably, by the aforedescribed wet oxalate coprecipitation process.In accordance with this method, a selected quantity of the calcinedoxalate powder (preferably milled) is pressed in a graphite die withmolybdenum foil used as spacers between the upper and lower graphiteplungers. Alternatively, a boron nitride coated graphite die may beused. A pressure of about 1000 psi to 1200 psi is applied at atemperature of between about 600° C. and 700° C. under a vacuum of lessthan 200 microns and maintained for about one hour. Thereafter, thepressure is increased to approximately between 4000 psi and 10,000 psiand the temperature increased to between 1300° C. and 1600° C. Thepressure is released after a hold at the elevated temperature of betweenone-half to four hours and the sample furnace cooled to roomtemperature.

Ceramic scintillator samples prepared in accordance with thehot-pressing method may be discolored due to surface reaction with themolybdenum spacer. Additional discoloration may be due to oxygendeficiency in the furnace atmosphere during hot pressing. The ceramics,however, can be made optically clear by oxidation in air or anoxygen-containing atmosphere at a temperature of about 800° C. to 1200°C. for between one and twenty hours. Any residual discoloration may beremoved by conventional grinding and polishing techniques.

In a specific example of the preparation of a scintillator material bythe vacuum hot-pressing method, 10 grams of a scintillator oxidematerial were obtained from the aforedescribed oxalate coprecipitationprocess by calcination of the oxalates at 800° C. for one hour in air.The oxides were initially hot pressed in a boron nitride coated graphitedie at 700° C. for one hour under a vacuum of about 20 microns and at apressure of 1200 psi. The temperature and pressure were then increasedto 1400° C. and 6,000 psi, respectively, under a vacuum of approximately100 microns. These conditions were maintained for two hours, followingwhich the pressure was released and the resulting scintillator materialfurnace cooled.

The scintillator material was gray to gray-black in color due to thereducing atmosphere created in the hot press. Light grinding of thescintillator surface and heating at 950° for four hours removed carbonsticking to the scintillator material. The remainder of the darkcoloration was removed by additional oxidation at 1150° C. for two hoursin air. The resulting scintillator material was "light tan" in color,translucent to transparent in optical quality, and exhibited goodrelative light output upon excitation by x-rays.

From the foregoing, it will be appreciated that the invention provides amethod for vacuum hot pressing transparent-to-translucent,rare-earth-doped polycrystalline yttria-gadolinia ceramic scintillatorshaving cubic crystalline structure, high x-ray stopping power, highradiant efficiency, high density, and high uniformity, as well as lowluminescence afterglow, and which are useful as radiation detectors suchas those used in CT and digital radiography.

While certain preferred features of the invention have been shown by wayof illustration, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

The invention claimed is:
 1. A method for preparing a polycrystallineceramic x-ray or gamma ray scintillator body comprising the stepsof:preparing a multicomponent powder consisting essentially of betweenabout 5 and 50 mole percent Gd₂ O₃, and between about 0.02 and 12 molepercent of at least one rare earth activator oxide selected from thegroup consisting of Eu₂ O₃, Nd₂ O₃, Yb₂ O₃, Dy₂ O₃, Tb₂ O₃, and Pr₂ O₃,the remainder of said multicomponent powder being Y₂ O₃ ; pressing saidmulticomponent powder under vacuum at a first temperature and pressurefor a first predetermined period of time; and PG,25 increasing saidfirst temperature and pressure to a second temperature and pressure andmaintaining said second temperature and pressure for a second period oftime to form said polycrystalline ceramic scintillator body, whereby theoptical clarity of said scintillator body is transparent-to-translucent.2. The method of claim 1 wherein said multicomponent powder comprisesbetween about 20 and 40 mole percent Gd₂ O₃, between about 1 and 6 molepercent Eu₂ O₃, and between about 0.1 and 2 mole percent Yb₂ O₃, theremainder being Y₂ O₃.
 3. The method of claim 1 wherein saidmulticomponent powder comprises between about 1 and 6 mole percent Eu₂O₃.
 4. The method of claim 1 wherein said multicomponent powdercomprises between about 0.05 and 1.5 mole percent Nd₂ O₃.
 5. The methodof claim 1 wherein said multicomponent powder comprises between about0.05 and 3 mole percent Tb₂ O₃.
 6. The method of claim 1 wherein saidmulticomponent powder comprises between about 0.1 and 2 mole percent Yb₂O₃.
 7. The method of claim 1 wherein said multicomponent powdercomprises between about 0.03 and 1 mole percent Dy₂ O₃.
 8. The method ofclaim 1 wherein said multicomponent powder comprises between about 0.02and 0.05 mole percent Pr₂ O₃.
 9. The method of claim 1 wherein saidmulticomponent powder comprises between about 20 and 40 mole percent Gd₂O₃.
 10. The method of claim 1 wherein said multicomponent powder furthercomprises between about 0.1 and 2 mole percent SrO.
 11. The method ofclaim 1 wherein said multicomponent powder further comprises betweenabout 0.1 and 2mole percent CaO.
 12. The method of claim 1 wherein saidstep of preparing said multicomponent powder comprises mixing highpurity, micron-to-submicron powders of Gd₂ O₃, Y₂ O₃, and at least oneof said rare earth activator oxides.
 13. The method of claim 1 whereinsaid step of preparing said multicomponent powder comprises the stepsof:coprecipitating by the wet chemical oxalate process the oxalates ofY, Gd, and at least one of said rare earth activator oxides; andcalcining said oxalates so as to obtain the corresponding oxides. 14.The method of claim 13 wherein said step of calcining comprises the stepof heating said oxalates in air at a temperature of between about 700°C. and 900° C. for between about 1 hour and 4 hours.
 15. The method ofclaim 13 wherein said step of calcining is preceded by the step ofmilling said oxalates.
 16. The method of claim 13 or 15 furthercomprising the step of milling said oxides obtained in said calciningstep.
 17. The method of claim 1, 12, or 13 wherein said step of pressingcomprises the step of pressing said multicomponent powder at a pressureof between about 1,000 psi and 1,200 psi at a temperature of betweenabout 600° C. and 700° C. under a vacuum of up to 200 microns for 1hour.
 18. The method of claim 17 wherein said step of increasing saidfirst temperature and pressure comprises increasing the temperature tobetween about 1300° C. and 1600° C., increasing the pressure to betweenabout 4,000 psi and 10,000 psi, and maintaining said increasedtemperature and pressure for between about one-half hour and 4 hours.19. The method of claim 18 further comprising the step of oxidizing theresulting hot pressed ceramic, said step of oxidizing including heatingsaid ceramic in an oxygen-containing atmosphere for between about 1 and20 hours at a temperature of between about 800° C. and 1200° C. so as toremove any discoloration.